Modern cellular telephones require ever higher data transmission rates and different mobile radio standards. While the GSM mobile radio standard already provides adequate data transmission rates for speech data transmission, the WCDMA/UMTS mobile radio standard is required for transmission of greater amounts of data. The W-LAN (Wireless Local Area Network) is used for very high data transmission rates, for example, for multimedia applications. Owing to the narrow available frequency spectrum, different transmission frequencies in the range from 800 MHz to 2.5 GHz are provided for the various mobile radio standards. The different requirements for the transmission and receiver stages in mobile communication appliances necessitate the development of radio-frequency assemblies of high functionality.
Examples of various assemblies or modules that are used in the transmission and reception paths of a communication appliance or cellular telephone, or cellphone for short, include not only the power amplifier for amplification of a signal to be transmitted as well as an antenna, but also an antenna switch, which connects antennas with different resonant frequencies to the power amplifier. Each antenna that is used within the cellphone is provided with its own matching to the power amplifier in the transmission path.
The same applies to the receiving amplifier, which is likewise coupled to the various antennas. The various assemblies or modules in the transmission path and in the reception path of a mobile communication appliance are in general each developed, designed and implemented in their own right. They are accommodated on different substrate mounts and are each provided with a standardized 50 ohm interface.
FIG. 8 shows a detail of a transmission path in a conventional communication appliance. In this case, a power amplifier P1 is connected to an antenna switch P2. One output of the antenna switch P2 in turn leads to an antenna, which is not shown here. The power amplifier P1 and the switch P2, which are formed in different semiconductor bodies, have the respective impedance Z1 and Z2. The two impedances Z1 and Z2 of the circuits differ from one another and correspond to the impedances of the circuits which process the signals. In order to minimize the signal losses resulting from reflections, the circuit A1 transforms the impedance to 50 ohms, upstream of the output of the power amplifier P1. The output of the power amplifier is then 50 ohms.
In the same way, a matching apparatus A2 is connected upstream of the internal signal processing for the antenna circuit P2, and transforms the line impedance from 50 ohms on the line L to the internal impedance Z2 of the antenna circuit P2. The power amplifier P1 and the antenna circuit P2 can thus be connected to one another, despite having different internal circuit impedances Z1 and Z2. The transformation circuits A1 and A2 are in this case generally accommodated within the corresponding semiconductor body of the power amplifier P1 and of the antenna circuit P2, respectively. Any mismatching of the line L, which would lead to greater reflection losses, is thus reduced.
The provision of the standardized 50 ohm interfaces allows widely differing assemblies or modules to be connected such that they are matched to one another at radio frequencies.
However, matching of the internal impedance Z1 to the input impedance of 50 ohms and, respectively, the output impedance, by the transformation circuit A3 and A1, respectively, is possible only within a relatively narrow frequency range. This is due to the internal matching circuits A3 and A1, respectively, which allow matching of the respective impedances Z2 and Z1 only in a narrow frequency range. Outside these frequency ranges, matching errors result in losses, which considerably reduce the performance of the corresponding component. Any change in the internal impedance Z1 likewise leads to a mismatch and to a discrepancy from the standardized 50 ohms, since the transformation circuits A1 and A3 now carry out fixed impedance matching. This effect is particularly evident in the case of antennas that are especially sensitive to environmental influences. When designing a transmitting arrangement, an appropriate margin must therefore be planned in, in order to make it possible to satisfy the entire specification. This increases the overall power consumption of the entire circuit, reduces the efficiency, and increases the production costs. Furthermore, a number of space-saving antennas must be matched to the geometry of the housing.